This invention relates to an improved molecular-sieve-catalyzed process for isomerizing an unextracted, ethylbenzene-containing xylene feed to a mixture rich in paraxylene which converts the ethylbenzene content primarily to benzene and ethane and provides high conversion of non-aromatic hydrocarbons in the feed to easily separable products. More particularly, this invention relates to a process for isomerizing an unextracted xylene feed containing a substantial quantity of ethylbenzene to a mixture rich in paraxylene over a supported, platinum-containing, medium pore size, crystalline, silicate molecular sieve catalyst composition under narrowly defined process conditions of temperature, total pressure, and hydrogen to hydrocarbon mol ratio in which the ethylbenzene present in the feed is largely converted to benzene and ethane by hydrodeethylation, and the primarily C.sub.9 paraffins and naphthenes (P/Ns) present are effectively removed by conversion to light hydrocarbons.
Typically, paraxylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by isomerization followed by, for example, lower-temperature crystallization of the paraxylene with recycle of the crystallizer liquid phase to the isomerizer. Principal raw materials are catalytically reformed naphthas and petroleum distillates. The fractions from these sources that contain the C.sub.8 aromatics vary quite widely in composition but will usually contain 10 to 35 weight percent ethylbenzene and up to about 10 weight percent primarily C.sub.9 paraffins and naphthenes with the remainder being primarily xylenes divided approximately 50 weight percent meta, and 25 percent each of the ortho and para isomers. The primarily C.sub.9 paraffins and naphthenes can be removed substantially by extraction to produce what are termed "extracted" xylene feeds. The extraction step adds to processing costs. Feeds that do not have the primarily C.sub.9 paraffins and naphthenes removed by extraction are termed "unextracted" xylene feeds.
The ethylbenzene in a xylene mixture is very difficult to separate from the other components due to similar volatility, and, if it can be converted during isomerization to products more readily separated from the xylenes, buildup of ethylbenzene in the recycle loop is prevented and process economics are greatly improved. The primarily C.sub.9 paraffins and naphthenes present in unextracted feeds unless removed also build up in the recycle loop and are usually extracted prior to isomerization as most commercial isomerization processes do not provide a catalyst which effectively converts them to easily separable-by-distillation products. Thus, it would be valuable to have a catalyst/process for xylene isomerization which would effectively convert both the ethylbenzene and primarily C.sub.9 paraffins and naphthenes to easily separable products without affecting the isomerization efficiency.
Xylene isomerization catalysts can be classified into three types based upon the manner in which they convert ethylbenzene: (1) naphthene pool catalysts, (2) transalkylation catalysts, and (3) hydrodeethylation catalysts.
Naphthene pool catalysts are capable of converting a portion of the ethylbenzene to xylenes via naphthene intermediates. These catalysts contain a strong hydrogenation function, such as platinum, and an acid function, such as chlorided alumina, amorphous silica-alumina, or a molecular sieve. The role of the hydrogenation function in these catalysts is to hydrogenate the C.sub.8 aromatics to establish essentially equilibrium between the C.sub.8 aromatics and the C.sub.8 cyclohexanes. The acid function interconverts ethylcyclohexane and the dimethylcyclohexanes via cyclopentane intermediates. These C.sub.8 cycloparaffins form the so-called naphthene pool.
It is necessary to operate naphthene pool catalysts at conditions that allow the formation of a sizable naphthene pool to allow efficient conversion of ethylbenzene to xylenes. Unfortunately, naphthenes can crack on the acid function of the catalyst, and the rate of cracking increases with the size of the naphthene pool. Naphthene cracking leads to high xylene loss, and the byproducts produced by naphthene cracking are low-valued paraffins. Thus, naphthene pool catalysts are generally less economic than the transalkylation-type and hydrodeethylation-type catalysts.
The transalkylation catalysts generally contain a shape selective molecular sieve. A shape selective catalyst is one that prevents some reactions from occurring based on the size of the reactants, products, or intermediates involved. In the case of common transalkylation catalysts, the molecular sieve contains pores that are apparently large enough to allow ethyl transfer to occur via a dealkylation/realkylation mechanism, but small enough to substantially suppress methyl transfer which requires the formation of a bulky biphenylalkane intermediate. The ability of transalkylation catalysts to catalyze ethyl transfer while suppressing methyl transfer allows these catalysts to convert ethylbenzene while minimizing xylene loss via xylene disproportionation.
When ethyl transfer occurs primarily by dealkylation/realkylation, it is possible to intercept and hydrogenate the ethylene intermediate involved with this mechanism of ethyl transfer by adding a hydrogenation function to the catalyst. The primary route for converting ethylbenzene then becomes hydrodeethylation, which is the conversion of ethylbenzene to benzene and ethane. It is desirable to selectively hydrogenate the ethylene intermediate without hydrogenating aromatics (without establishing a naphthene pool) to prevent the cracking of the naphthenes that occurs over the acid function of the catalyst. Commercial hydrodeethylation catalysts selectively hydrogenate ethylene without substantial hydrogenation of aromatics at reported commercial conditions. At these same conditions, a small amount of impregnated platinum compound will allow substantial hydrogenation of aromatics.
In order to form a hydrodeethylation catalyst, it is essential to use an acidic component that behaves as a shape selective catalyst, i.e., one that suppresses the formation of the bulky biphenylalkane intermediate required for transmethylation, because transethylation can occur via a similar intermediate. For catalysts with pores large enough to allow the formation of these biphenylalkane intermediates, transethylation appears to occur primarily via these intermediates. In this case, ethylene is not an intermediate for transethylation, and the addition of a hydrogenation component cannot produce a hydrodeethylation catalyst.
Now it has been found that by choosing isomerization temperature, total pressure, and mol ratio, hydrogen/hydrocarbon, in narrowly defined ranges, a platinum-containing, acidic, medium pore size, molecular sieve catalyst composition can be used in a process in which most of the ethylbenzene is removed by hydrodeethylation, and the size of the naphthene pool is reduced resulting in less xylene loss. In addition, it has been found that this catalyst and certain sets of temperature, total pressure, and hydrogen to hydrocarbon mol ratio variables within this narrowly defined set of process condition ranges provide very high conversion of paraffins and naphthenes to light products which can be readily separated from the reactor effluent. Thus, ethylbenzene is removed by a particularly attractive process, a greater amount of paraffins and naphthenes can be tolerated in the isomerizer feed, and the xylene isomerization effectiveness is essentially unchanged.